Apparatus for reading microfilm or michofiche, also generally known as microfilm readers or viewers, permit to magnify printed or illustrated documentation and other material stored in a considerably reduced size as compared to the original size, such as files, catalogs, documents and other data of varied sorts, such that they are reproduced in a readable format.
The use of microfilm and the like and of the accompanying reproduction equipment accomplishes the result of greatly reduced space requirements for storage of the information data as well as improved speed of access to the information. Most of the microfilm reading units presently on the market are of the rear projection screen type. Such viewers are provided with an incandescent lamp with an appropriate optical condenser system for illuminating the microfilm with intense light, with an objective lens system for enlarging the image in focus and projecting the image on the translucent rear surface of a projection screen. In this manner, the image projected on the translucent screen is viewed through the screen. An enlargement to at least 70% of the original size is necessary to reproduce the information stored on the microfilm to a readable format without eye strain. This, in turn, necessitates the use of a screen of such dimension that microfilm readers are generally in the form of table models, resulting in a corresponding large space requirement. Portable microfilm readers have been developed which are even collapsible in some cases. Even when collapsible, portable units are relatively large and heavy and they require a multiplicity of handles for their operation. A further disadvantage of conventional microfilm reading units is that ambient lighting decreases considerably the contrast of the image on the screen. A particular disadvantage of conventional microfilm readers is that they are dependent, as a rule, on electrical current outlets.
In order to alleviate some of those disadvantages, smaller pocket viewers have been designed which function according to a magnifying glass principle using daylight or lamps and batteries for illumination of the microfilm. Such microfilm viewers, however, are monocular and are limited in their applications to a magnification of about 24X. In German Pat. No. 1,907,975 there is disclosed a microfilm viewer of this type of means of which the user merely views an illuminated transparency through a simple lens. The transparency is illuminated by means of an electric light bulb disposed behind the transparency, and which is mounted on a separate portion of the housing designated as the housing bottom. If the "bottom" of the housing, including the light bulb and the batteries, is removed, light is allowed to enter through the opening for illumination of the transparency. Consequently, in the apparatus disclosed in the aforesaid German patent, a change from internal to external illumination of the transparency and vice versa, necessitates a regular dismantling and reassembling of the housing of the transparency viewing unit. In addition, in such an arrangement as the one described, daylight impinges simply and directly upon the transparency. Therefore, when daylight illumination is used, sufficient illumination of the transparency is not always available.
By contrast to the prior art microfilm viewers, the present invention provides a viewer operating according to a microscope principle, i.e., provided with one objective lens system and a pair of eyepieces. A binocular microfilm reader is thus provided in which, to insure greatly improved compactness of the unit, the light beam between the object and the image at the eyepieces is subjected to multiple reflection by means of mirrors. Such an arrangement provides the 45X magnification of the image stored on the microfilm which is a present day requirement. In addition binocular viewing, as compared to monocular viewing, presents the advantage of being far less tiring to the eye and capable of producing an image noticably improved.
German Pat. No. 2,120,329 discloses a microscope in which illumination of the stage is effected by either an external or an internal light source. However, the light must in either case be projected onto the object at a 90.degree. angle by means of a reflecting surface. Furthermore, the light beam provided by the external source is directed in the direction opposite to that of the light beam provided by the internal light source, namely from the side and in a direction perpendicular to the axis of the single eyepiece. Such an arrangement would be particularly disadvantageous in portable microfilm viewers since it is basically difficult to find a laterally located external light source. The stage illumination unit disclosed in the aforesaid German patent, which is arranged for internal as well as external light source, has a V-shaped reflector for deflecting the light at an 90.degree. angle, which must be correctly adjusted to at least two switching positions. Such movable reflectors with a pair of inclined mirrors do not only represent increased manufacturing and assembly costs, but they also cause a decrease in the effectiveness and the trouble-free operation of the illumination unit, and it also makes the general operation of the device during switching from the internal to the external light source much more difficult. Due to the necessary reflection of the impinging light during internal illumination it is necessary, for the purpose of providing sufficient illumination of the stage, to use a light bulb with a built-in condenser, in which the light is radiated mainly in the direction of the reflecting surface facing the light bulb. Such a condensing light bulb, however, is a special bulb which is much more expensive than regular light bulbs and is, in addition, not always readily available on the market. Furthermore, the light from the external source enters the optical system without prior convergence and is only thereafter reflected onto the stage by the second inclined reflector surface. With internal as well as external illumination less than optimal light density falls onto the inclined reflector surfaces which, in turn, do not concentrate the full amount of light which they receive for transmitting to the object. It must also be kept in mind that the inclined reflector surfaces, and especially the one facing the external light source, become relatively dirty in a very short period of time. It is readily apparent that the utilization of the light at the site of the object is less than optimum in quality. Also, operation of the illumination unit as well as correct adjustment of the reflectors and of the condensing light bulb, which obviously requires dealing with intermediate adjustments, is quite difficult, especially in the dark.
The present invention, by contrast, provides an illumination unit for a microfilm reader or viewer working according to a binocular microscope principle and arranged in such a manner that the reader or viewer does not exhibit the aforementioned disadvantages of prior art instruments and, most of all, which in spite of a simple and effective design of the illumination unit has an overall optimum compactness and many other applications.
The present invention provides an illumination unit particularly well suited for illuminating a microfilm in a microfilm viewer based on the binocular microscope principle, which is designed with simplicity in mind and fulfills, through its advantageous arrangement of the various elements, the most sophisticated requirements which modern technology demands of such units. It must first be pointed out that the invention achieves, with particularly simple means, an optimum illumination of the microfilm to be viewed, irrespective of whether an external or an internal light source is used, especially since the light, in both cases, reaches the object without any unnecessary reflection. Consequently, the light obtained from an external light source is concentrated by a focusing and condensing lens in such manner that it is of sufficient density to illuminate a large area of the microfilm to be viewed. In addition, the internal light source may be in the form of an ordinary light bulb. The focusing and condensing lens is in the form of a Fresnel lens, and this also contributes to the compactness of the unit and to its potential for high power magnification. The compactness and the other qualities of the portable viewer according to the present invention gives it a wide variety of applications, for example, at building sites, where a large number of blueprints, parts lists, specifications and other data, previously recorded on microfilm, may be read without any problem. Further applications of the portable viewer of the invention may be found in the technical fields, such as operational units of the army, and like applications. Switching from one type of illumination to the other is particularly simple in the viewer according to the present invention. The advantage provided by mounting the condenser lens in the wall of the viewer housing, whereby light from the internal source is prevented from shining on the outside, is particularly advantageous for military night time operations in which light could result in an immediate discovery of easily identifiable targets. Finally, it will be readily appreciated that operation of the viewer according to the present invention is particularly convenient since the light from an external source reaches the microfilm directly, that is, without any prior reflection. Thus, it is possible to use the portable compact unit in almost the same manner as a pair of field binoculars, and it is readily apparent that an external light source can be found without any difficulty.
U.S. Pat. No. 3,797,917 discloses a transparency viewer utilizing a Fresnel lens through which the user of the viewer sees an enlarged image of the transparency such that as a diapositive or the like. However, the structure does not include a closed housing between the Fresnel lens and the transparency, and no switching from external to internal light source is available for this device. One of the characteristic differences between this device and the present invention is that the Fresnel lens in the transparency viewer of the prior art serves no purpose other than that of a simple magnifying glass, and is disposed between the transparency holder and the observer. By contrast, the Fresnel lens used in the present invention has for funcion to concentrate the light from the external light source onto a scattering or diffusion screen located in front of the microfilm. Consequently, the Fresnel lens is located behind the microfilm relative to an observer of the microfilm.
A further advantage of the present invention results from the internal illumination light bulb being fastened to a U-shaped contact housing attached turn to a pull lever U-shaped handle, the contact housing being provided with a lateral light bulb socket and a spring biased blade electrical contact, with the result that the contact housing, on its upper side, by means of a cover also provided with a contact in the form of a spring action contact arm is electrically connected to one pole of a battery and by pulling out the pull lever handle is automatically connected to the other pole of the battery means of a spring blade contact located beneath the contact housing. This particularly simple switching mechanism permits, through a single pulling action on the pull handle a direct switch from external to internal light source for illumination of the microfilm. In like fashion, the light bulb may be inserted without difficulty into the plug-in socket, whereby a second spring loaded electrical contact is positively pressed against the base terminal of the light bulb.
Advantageously, the contact housing in the switch-on position is stopped by a projection portion engaging a notch in the contact housing. Thus, the switching-on of the light may be effected positively and with safety even in the dark without having to read any position markings. In addition, there is assurance that, especially under adverse conditions, an unvoluntary switching of the internal light cannot take place. A further advantage of the invention is that a cover plate is provided for the condenser lens which engages against an arresting abutment in the housing wall when the contact housing is displaced in the switch on position. In addition, the lens cover is coated, on its side facing the internal light source with a light reflective coating.
This cover plate, coated with a reflective coating, thus forms in a very unique way a reflector of its own, such that upon turning on the internal light bulb an even better illumination of the microfilm is provided. The cover plate, therefore, has a double function.